Takes an already connected socket and starts server side TLS
handshake to make the communication secure. When the returned
future completes the RawSecureSocket has completed the TLS
handshake. Using this function requires that the other end of the
connection is going to start the TLS handshake.

If the socket already has a subscription, pass the existing
subscription in the subscription parameter. The secureServer
operation will take over the subscription by replacing the
handlers with it own secure processing. The caller must not touch
this subscription anymore. Passing a paused subscription is an
error.

If some of the data of the TLS handshake has already been read
from the socket this data can be passed in the bufferedData
parameter. This data will be processed before any other data
available on the socket.